Fix return type of IteratorIterator::getInnerIterator

[alcaeus]

When creating an IteratorIterator with an iterator of TIterator, we can assume that getInnerIterator will return that same TIterator type. Technically, this change could be applied to OuterIterator, although that would require us to introduce a TIterator template to OuterIterator, which may break existing usages beyond the default iterators for which stubs are provided. Please advise if it's ok to change OuterIterator to get an additional template for the iterator it's wrapping.